This application claims the priority of Korean Patent Application No. 2002-78146, filed on Dec. 10, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a polarization mode dispersion (PMD) compensator for automatically and rapidly compensating PMD occurring in an optical transmission fiber in a high-speed optical transmission system, and a PMD compensating method.
2. Description of the Related Art
Since pulse dispersion occurring in optical transmission fibers limits a transmission bandwidth and a transmission distance, it is an obstacle to develop high-capacity, long-haul optical communication systems. Chromatic dispersion and PMD are main causes of such pulse dispersion. Since chromatic dispersion is predetermined and does not vary with time, color dispersion can be completely compensated for. PMD has statistical characteristics resulting from birefringence and random polarization mode coupling appearing in an optical transmission fiber due to internal and external factors and varies with time. Accordingly, a fast real-time automatic adaptive compensator faster than the variation speed of PMD is required to compensate for PMD.
The main principle of a PMD compensator is to adjust two orthogonal principal states of polarization (PSP) in an optical transmission fiber and a differential group delay (DGD) between the two PSP using a polarization controller and a time delay line controller to remove a differential time delay between the two PSP. Examples employing such a principle of compensating PMD are disclosed as follows.
A PMD compensation method is disclosed in the article entitled “Polarization Mode Dispersion Compensation by Phase Diversity Detection” by B. W. Hakki and published in IEEE Photonics Technology Letters, vol 9, pp. 121–123, 1997. In this disclosed method, a compensator finds a differential time delay between two PSP from two mixers and applies a time delay having the same size as and an opposite direction to the differential time delay to a variable delay line in order to compensate for PMD. Here, electric signals input to the two mixers should be equal regardless of a variation in the magnitude of optical power of the two PSP. A separate high-speed automatic gain control (AGC) circuit is necessary for this. As a result, an additional high-priced, high-speed sophisticated circuitry as well as mixers is needed as a bit rate increases.
R. Noe et al. revealed the article entitled “Polarization Mode Dispersing Compensation at 20 Gib/s with Fiber-based Distributed Equalizer” and published in Electronics Letters, vol. 34, pp. 2421–2422, 1998. In this article, polarization transformers located among several pieces of polarization maintaining fiber (PMF) are controlled from linear combinations of power components of several specific electric frequencies to eliminate a differential time delay between two orthogonal polarizations due to PMD. Accordingly, this method has disadvantages in that compensation for PMD takes a long time and control of a time delay is discontinuous due to the adjustment of several polarization transformers.
U.S. Pat. No. 5,930,414 discloses “Method and Apparatus for Automatic Compensation of First-Order Polarization Mode Dispersion (PMD)” in which a Mach-Zehnder interferometer type compensator integrates an electrical spectrum output therefrom and one controller monitors the integrated value to alternately feed an optical delay line and a polarization transformer back in order to compensate for a differential time delay due to PMD. In this method, all polarization states have to be changed by adjusting the polarization transformer to each delay value given by the optical delay line to find a maximum integration value of the electrical spectrum. Thus, compensation for PMD takes a long time.